Semiconductor memory devices are typically classified into volatile memory devices and non-volatile memory devices. Volatile memory devices are subdivided into dynamic random access memories (DRAMs) and static random access memories (SRAMs). Non-volatile memory types include erasable programmable read-only memories (EPROMs) and electrically erasable programmable read-only memories (EEPROMs). EEPROMs are increasingly used in system programming that requires continuous update or auxiliary memory devices. Particularly, flash EEPROMs are advantageous as mass storage devices because their integration density is high compared with conventional EEPROMs.
Frequently, it would be convenient to be able to mix integrated circuit device types, such as EEPROMs and other memory devices, and bipolar integrated circuits, such as NPN transistors, onto a single integrated circuit chip. However, due to the inherently low breakdown voltage (approximately 10 volts) of typical wells used in BiCMOS technology and the need for a high programming voltage of an EEPROM memory device (approximately 14 volts), there has been no simple and economical way to integrate these two device types into a single integrated circuit. Previously, the problem has been avoided in the art by using additional masks to create high-voltage wells for EEPROMs.